Improvements of the rigidity of tread part in a pneumatic tire have been attempted by forming a belt layer comprising belt plies outside the carcass in the tread part. The rigidity reduces toward the shoulder part from the center in the tire's equator. In other words, as the tread width becomes wider and the aspect ratio becomes lower, the tread radius tends to be flatter, and the cornering force of the tire is increased.
It is known that the rigidity of the belt layer is remarkably increased because of a hoop effect by overlaying plural belt plies with cords placed in different directions.
On the other hand, the shape of tread part is significantly changed at the shoulder part in driving. Therefore, both ends of the belt plies are extended near to the shoulder part to control such change in shape and avoid separation and deviational wear of the belt ply ends.
The rigidity of tread part can be improved by the structure.
However, since the cords of belt plies are crossed, the hoop effect between the belt plies is strengthened. Moreover, as plural belt plies are united, they are stiffened. Therefore, when the aspect ratio is reduced, the tread surface "a" tends to have a reverse warp "r" as shown in FIG. 7, and, as a result, the high-speed durability is reduced, vibrations increase and the riding comfort is reduced.
In addition, as for the distribution of ground-contact pressure, shoulder parts come to have a higher pressure than the central part. As a result, a deviational wear occurs between the shoulder parts and central parts, which becomes a problem by reducing the durability.
A tire having an aspect ratio of 70% or less, specifically, is superior in ground contact performance, while the aforementioned tendency is observed significantly. In the case of forming belt plies by using steel cords, since the rigidity of belt plies themselves is high, the reverse warp in the tread surface becomes pronounced.